Previous studies have shown that spending more time outside can result in a variety of health benefits ranging from protecting against depression, diabetes, obesity, ADHD, cardiovascular disease, cancer and many more. Now the University of Illinois can explain why this is the case.

"I pulled every bit of the research in this area together that I could find and was surprised to realize I could trace as many as 21 possible pathways between nature and good health – and even more surprised to realize that all but two of the pathways shared a single common denominator," Ming Kuo, a University of Illinois environment and behavior researcher, said in a news release.

Kuo found that when people spend more time outside, the body enters the "rest and digest" mode, opposed to the "fight or flight" mode. The "rest and digest" mode allows the body to relax and invest more resources into maintaining the immune system, which is given lower priority when the body is "fight or flight" mode that stree can mimick.

"The realization that there are so many pathways helps explain not only how nature promotes health, but also why nature has such huge, broad effects on health," Kuo explained. "Nature doesn't just have one or two active ingredients. It's more like a multivitamin that provides us with all sorts of the nutrients we need. That's how nature can protect us from all these different kinds of diseases -- cardiovascular, respiratory, mental health, musculoskeletal, etc. -- simultaneously."

Similar immune boosts are also associated with relaxing indoor activities, such as board games. However, being indoors eliminates other benefits, such as recieving vitamin D from sunlight, for example.